1. Field of the Invention
The present invention relates to the field of heating, and more particularly, to the use of microwave radiation for heating slab-like layers or surfaces.
2. Description of the Related Art
The use of microwave radiation is a well known method for heating substances that have intrinsic absorption properties, but it is often difficult to remove the effects of cavity and waveguide modes that lead to non-uniform heating and “hot spots” in the target to be heated.
Many processes also require uniform heating and a method of applying heat energy noninvasively. For example, the use of microwave heating has been proven to be effective for the processing of dielectric material. In many cases, a uniform temperature distribution within the product is required.
There have been many proposals that use a TEM waveguide mode to create a uniform field distribution. A waveguide or cavity is loaded with high permittivity dielectric materials to enable the uniform TEM mode field distributions. The disadvantage is that many applications do not allow the inclusion of such material within the processing environment. Another disadvantage is that the loading material limits the space available within the cavity for the target.
Techniques have also been introduced that require a moving structure or a field enhancing structure within the interior of the heating cavity. In many cases, the added complexity is undesirable, such as, for example, a conveyer belt system moving over microwave emitting slots in a waveguide.
Other heating methods employ additional electrical structures within the cavity that alter the field distribution, such as, for example, an inserted control element positioned between an object being heated and a source of microwave radiation and employed to prevent a localized concentration of microwave energy resulting from a discontinuity in the object surface. However, close control is needed for heating an object with a sensitive coating.
High-frequency microwave sources have been proposed to reduce the spatial dimension of field variation and to facilitate the efficacy of multimode methods for time-averaged field uniformity, such as a 28-GHz source used for achieving uniformity within a small volume. This methodology relies on the disadvantage imposed by fixed-frequency microwave heating cavities that are known to have cold spots and hot spots. Such phenomena are attributed to the ratio of the wavelength to the size of the microwave cavity. With a relatively low frequency microwave introduced into a small cavity, standing waves occur and, thus, the microwave power does not uniformly fill all of the space within the cavity, and the unaffected regions are not heated. In the extreme case, the oven cavity becomes practically a “single-mode” cavity. At 2.45 GHz a far better uniformity of field can be obtained by increasing the cavity dimensions better than 100 times the wavelength which would require a cavity size of about 12 m. However, at this size a very large power supply would be required to produce a reasonable energy density within the cavity.
A proposed solution to the large power supply problem has been to go to higher frequencies, as high as 28 GHz where 100 times the wavelength is approximately 1 m in size. This is a far more manageable size of cavity and a reasonable energy density can be obtained with a moderate power source. However, a frequency of 28 GHz is considered to be prohibitively expensive for commercial use.
Hybrid heating ovens that incorporate airflow with the microwave heating are known to increase uniformity via convective heat transfer. However, in many cases, the increased complexity of introducing the airflow is prohibitive, or the desired process may be degraded by airflow.
In another method, a central conductor is imposed within a waveguide heating cavity to create the TEM field distribution. The central conductor is used as an air flow device to help unify the heating. However, many applications will not allow a central conductor within a heating cavity, such as, for example, a home microwave oven. In addition, TEM modes created in coaxial structures have electric fields that are non-uniform, falling off as the inverse of the distance from the axial conductor.
Attempts have also been made at mode stirring, or randomly deflecting the microwave beam, in order to break up the standing modes and thereby fill the cavity with the microwave radiation. One such attempt is the addition of rotating fan blades at the beam entrance of the cavity. This is essentially an empirical, non-deterministic technique based on statistical fluctuations in mode patterns. In cases where the cavity size is not large compared to a microwave wavelength, the number of modes available to be stirred is small and the statistical averaging in ineffective. These methods also rely on the inclusion of mechanical or electronic devices required to operate within the high-field, high-temperature processing environment. In many applications, this is undesirable.
A further method extending the deflecting approach involves the use of a circular cylindrical geometry where a bellows-type device is used to change the cavity's electrical length. By rapidly oscillating the length, many modes can come to bear on the sample and average out the heating to be more uniform. However, this requires a highly over-moded cavity and a complicated moving mechanical structure.
Another general method used to overcome the adverse effects of standing waves is to intentionally create a standing wave within a single-mode cavity such that the target may be placed at the location determined to have the highest power (the hot spot). Thus, only the portion of the cavity in which the standing wave is most concentrated will be used. This requires that the heating target is small compared to the cavity size and/or the mode structure cannot be altered from one target to another. It also does not lend itself to mass production, since other microwave cavity tuning devices, such as tuning stubs, are necessary for tuning the cavity for the desired mode. If the dielectric properties of the target change as it heats up, then the cavity resonance properties will also change, and the field distributions will also change in time.
Multiple microwave power sources and variable-frequency microwave sources are other solutions that have been proposed. The uniformity achieved through these approaches is dependent on having a statistically large number of modes available within the cavity, and they will work best when the cavity size is large compared to a wavelength. However, they impose a cost disadvantage. While 2.45 GHz/2 kW sources are very inexpensive and plentiful, any deviation from these parameters requires custom fabrication. A variable frequency source is potentially inexpensive at low power (e.g. a VCO), but they require high-power microwave amplifiers (>1 kW), which are virtually nonexistent for less than a few hundreds of thousands of dollars.
Other techniques have been proposed to move the target around within the cavity. The disadvantages here are that a mechanical device is necessary to move the target, and the target only can occupy a small portion of the cavity.
The use of meta-structures or artificial electromagnetic materials has yielded methods for creating uniform fields within a microwave waveguide or cavity, such as by a rectangular waveguide that utilizes a hard electromagnetic surface (HES) to enable TEM waves in a waveguide which is applied to an active amplifier array structure for the purpose of high-frequency amplification for communication purposes. A uniform field distribution is desired in this case because it optimizes the amplifiers performance and efficiency. In this regard, U.S. Pat. No. 6,603,357 discloses a rectangular waveguide that utilizes a hard electromagnetic surface to enable TEM waves in a waveguide. It is applied to an active amplifier array structure for the purpose of high-frequency amplification for communication purposes. However, the prior art meta-surfaces are complicated by one or more of the following issues. They either require (1) vias that ground the surface to the waveguide walls, (2) intricate patterns, (3) very high permittivity materials within the structure or they require active materials, or (4) dimensions that are not small compared to the waveguide size.
Therefore, a need exists for a better way of providing uniform microwave heating. Embodiments of the present invention provide solutions to meet such need.